Conventional photocathodes are used in a number of photon amplifier applications including photomultipliers (PMTs), microchannel-plate (MCP) amplifier tubes and Digicons. The photocathode detects the photon and emits an electron. The electron (i.e., the photoelectric current) is amplified by one of the previously mentioned technologies and the resulting signal is larger than the sensor noise. This amplification generally eliminates the need for extensive noise filtering and allows high-speed photon counting and energy-discrimination operations. However, conventional photocathodes and newer transferred electron photocathodes have relatively low quantum efficiencies and respond to photons in a very limited spectral range around the visible spectrum, from the long wavelength ultraviolet to the near infrared: wavelengths from about 0.1 microns to 1.7 microns. Quantum efficiency in this context refers to the average number of electrons emitted per incident photon of a given wavelength. PMTs, a particular electron amplifier using dynodes, are applied in a number of medical and visible-laser detection applications. Microchannel Plate (MCP) amplifier tubes are used in similar applications but because of their multiple-pixel imaging capability they are also used in night-vision goggles and imaging laser radar (LADAR). Night-vision MCP image tubes are vacuum structures containing a photocathode, a microchannel plate and a phosphor. Microchannel-plates amplify the photocathode electrons produced by dim-light photons, by collisions with the glass walls of the MCP, and these electrons, in turn, produce increased levels of visible light via collision with the phosphor. In photon counting applications, a microchannel plate or photomultiplier tube increases the single-photon signal level above the sensor noise, thereby increasing sensitivity of the sensor to the level at which photon counting can be performed. In a digicon the electron emitted by a photocathode is guided by a magnetic field and accelerated by an electric field, to energies of thousands of electron volts. The photoelectron impacts a silicon diode array and amplification results by impact onization which requires only about 3.3 eV to produce an lectron-hole pair in silicon.
Currently there are no photocathodes for x-rays, wavelengths shorter than about 0.01 microns. Therefore x-ray signal amplification first requires the conversion of x-rays to photons in the visible spectrum using scintillators or phosphors. These visible photons can then be amplified by conventional photocathode technologies discussed above. The problems with this amplification method stem from the inefficient conversion of x-rays to visible-light photons and with the poor quantum efficiencies of the photocathodes that can detect the scintillator-produced photons. Typically 50 eV of x-ray energy is required for a visible photon and photocathode quantum efficiencies are below 20%. Thus about 250 eV is required for each visible photon detected or for each electron emitted from the photocathode. In contrast the photocathode of the present invention would require only about 3.3 eV for each electron emitted. Furthermore the statistical noise of the photocathode of the present invention is less than a scintillator because the individual ionization events in the present invention are not completely independent.
There are many applications which require the discrimination of x-ray or gamma-ray energy. Nuclear medicine requires the discrimination of non-scattered gamma-rays from scattered gamma-rays to determine the absorption of radioactive compounds in tissue with sensors outside the body. The more accurately the energy discrimination can be done the faster the analysis can be achieved. However, the inefficiency of scintillator, gamma-ray-photon to visible-photon conversion limits the accuracy of energy discrimination. Energy resolution increases as the energy required to obtain a photocathode electron decreases. Energy resolution is also related to the statistics of x-ray or gamma-ray photon detection.
High-count-rate x-ray detectors are important in many areas of science where there is a Large background and high signal-to-noise ratio is obtained by energy discriminating the signal from the background. Generally solid-state detectors are used instead of scintillators to achieve this accuracy because of the high efficiency of conversion of x-ray energy to electron-hole pairs.
Solid-state detectors do not normally include amplification so signal filtering is required to reduce noise and limit uncertainty. This filtering, however, reduces the speed of operation, limiting the flux of photons that the sensor can process without saturation. Application examples are the non-invasive high-speed quantitative measure of lead in bones and other elements in other organs. (I. L. Preiss and M. A. Tariq On the use of L x-ray fluorescence for bone lead evaluation, Radiocanal. Nucl. Chem. Let. 164 (6), 381-385 (1992), I. L. Preiss and T. PTAK, Trace Element Profiles of Biological Samples Using Radioscope X-ray Fluorescence, Nuclear Instruments and Methods in Physics search A242 (1986) 539-543.) Data collection is particularly limited in structural biology investigations of dilute samples where detectors have not kept pace with synchrotron source development. The Extended X-ray Absorption Fine Structure (EXAFS) technique, counting fluorescent x-rays, has been known for some time, for example, but the counters have very limited data rates which cannot adequately take advantage of current and future synchrotron source fluxes.(J. Jaklevic et al, Solid State Communications, 23, 679 (1977)) Many samples have low concentrations of the element of interest which is embedded in a matrix of energy absorbing molecules. Under these conditions conventional detectors expend their count rate separating the desired-element-fluorescence x-rays from the larger number of quasi-elastic and matrix-fluorescence x-rays. A widely used x-ray fluorescence-detected spectroscopy detector is the 13-element Canberra Ge Detector (S P. Cramer et al, A 13-Element Ge Detector For Fluorescence EXAFS, Nuclear Instruments and Methods in Physics Research, A266, D586 (1988)). Cramer et al shows the count rate of a single detector element, using a 1 .mu.s shaping time (electronic noise of about a few hundred eV) corrected for dead time losses, is about 2.times.10.sup.5 photons/sec). This count rate is inadequate for many synchrotron-based experiments and the energy resolution is far from optimal.